dm is like gm, but faster and with fewer features.

This is sort of the near-minimal proof-of-concept skeleton.

  - It can run existing GMs.
  - It supports most configs (just not PDF).
  - --replay is the only "fancy" feature it currently supports

Hopefully you will be disturbed by its speed.

BUG=
R=epoger@google.com

Review URL: https://codereview.chromium.org/22839016

git-svn-id: http://skia.googlecode.com/svn/trunk@11802 2bbb7eff-a529-9590-31e7-b0007b416f81

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+DM is like GM, but multithreaded.  It doesn't do everything GM does yet.
+
+Current approximate list of missing features:
+  --mismatchPath
+  --missingExpectationsPath
+  --writePath
+  --writePicturePath
+
+  --deferred / --pipe
+  --rtree
+  --serialize
+  --tiledGrid
+
+
+DM's design is based around Tasks and a TaskRunner.
+
+A Task represents an independent unit of work that might fail.  We make a task
+for each GM/configuration pair we want to run.  Tasks can kick off new tasks
+themselves.  For example, a CpuTask can kick off a ReplayTask to make sure
+recording and playing back an SkPicture gives the same result as direct
+rendering.
+
+The TaskRunner runs all tasks on one of two threadpools, whose sizes are
+configurable by --cpuThreads and --gpuThreads.  Ideally we'd run these on a
+single threadpool but it can swamp the GPU if we shove too much work into it at
+once.  --cpuThreads defaults to the number of cores on the machine.
+--gpuThreads defaults to 1, but you may find 2 or 4 runs a little faster.
+
+So the main flow of DM is:
+
+    for each GM:
+        for each configuration:
+            kick off a new task
+    < tasks run, maybe fail, and maybe kick off new tasks >
+    wait for all tasks to finish
+    report failures
+